Because normal speech signals include much redundancy, as well as intervals without any information, an increased utilization of the transmission capacity of available channels can be achieved by a reduction of the unnecessary signal components. As a possible result fewer multiplex channels, i.e. time slots in periodic time frames, are required on a trunk line than the total number of speech signals that must be transmitted.
Numerous methods and arrangements for redundancy reduction are known in the art. In one of these methods, a trunk channel is only assigned for the duration of one period (e.g. 6 ms) to those of all connected voice input channels on which presently an "active" speech signal is received. As soon as a talker becomes silent the input channel is deprived of the trunk channel. Systems in which this method is used are disclosed in U.S. Pat. Nos. 4,032,719, 4,009,343 and 3,664,680, and also in a publication by K. Bullington, et al., "Engineering Aspects of TASI," Bell System Technical Journal, March, 1959, pp. 353-364. One problem dealing with the methods employing activity-controlled compression is the so-called "freezout", occuring during very high system utilization which means that to some of the speech signals, temporarily no trunk channel can be assigned despite an active status. Furthermore, delays can occur when a trunk channel is to be newly assigned after a pause. These phenomena may finally result in a reduction of speech quality. Furthermore, activity detectors are required for all input channels when this method is employed.
Another possibility for a reduction of the required transmission channel capacity is as follows: In the coded transmission of speech signals those samples are omitted, which are equal or very similar to the last-previous transmitted sample, and in the receiver the last-previous transmitted sample is used once again or even several times. Methods employing this principle were disclosed in the following publications:
R. Woitowitz: Ein Redundanzminderndes Sprachmultiplexverfahren Mit Momentaner Prioritatszuteilung. Proceedings, 1974, International Zurich Seminar on Digital Communications, Paper A3.
J. A. Sciulli et al.: A Speech Predictive Encoding Communication System for Multichannel Telephony, IEEE Transactions on Communications, Vol. COM-21 (1973), pp. 827-835.
Means for detecting equality or similarity of single coded samples, and for storing and reinserting previous samples in the receiver employing these techniques, are relatively simple. Since, however, omission and reinsertion is effected for single samples only, the signalling information which is required for utilization of the released time slots must be transmitted in each sampling interval in the form of assignment masks or addresses. This requires a significant portion of the channel capacity (in one case 25%) so that the gain in transmission capacity which is possible by redundancy reduction is drastically reduced. Furthermore, signals having large signal amplitudes are preferred by these methods which are based on the handling of single samples.
The paper "Adaptive Predictive Speech Coding Based on Pitch-Controlled Interruption/Reiteration Techniques" by A. Frei et al., published in the Proceedings of the IEEE, 1973, International Conference on Communications, pp. 46-12 through 46-16, discloses a method in which, for the transmission of voiced portions of a speech signal, only one pitch period is transmitted as delta-coded signal, and in which this signal section of variable length is stored in the receiver in a shift register and is repeated several times for obtaining the output signal. The required information for the pitch period is obtained by a specific correlator which compares a delta-coded signal subsection (window) with a delayed subsection of the same signal. (Such a pitch period detector is also described in Swiss Pat. No. 549,849.) This transmission method has the disadvantage that sections of variable length must be handled, because the pitch period and thus the length of the signal interval to be transmitted as well as the working length of the shift register are varying. This fact can be tolerated for the transmission of single signals, but not for the time division multiplex transmission of speech signals in synchronous operation (time frames of fixed length). Other disadvantages are the occurence of echo-like effects at high repetition rates and of rumble distortions due to transient effects when the pitch frequency drifts.